Metallogeny of the Fairbanks Mining District, Alaska and adjacent areas

Abstract

The Fairbanks mining district encompasses an area of 1500 km2 (600 miles2) centred just north of the City of Fairbanks, Alaska. The district is one of six mining areas located in or near the northwestern margin of the Yukon-Tanana Uplands of east-central Alaska and the Yukon Territory, Canada. The six mining districts in Alaska (Fairbanks, Circle, Steese, Richardson, Tolovana and Kantishna) and the Klondike district nearby in the Yukon Territory, have an aggregate placer gold production of 25 million troy ounces. This production establishes the region as one of the largest gold producing areas of North America. The aim of the present investigation is to define, classify and explain the genesis of the several primary sources from which the placer gold deposits of the region were derived. Through geological mapping and sampling of the districts, the 350 identified primary mineral occurrences are classified into eight categories as follows: (1) metamorphosed volcanic-exhalative and associated low-sulfide Au-quartz veins, (2) Cu-Mo-Au porphyries, (3) precious metal enriched massive sulfides, (4) epithermal veins in plutonic rocks, (5) Au-bearing tungsten skarns, (6) Sn greisen-gold-quartz veins, (7) Sediment-hosted gold of the Carlin type, and (8) palaeoplacer gold deposits. Geological mapping and sampling has also established that recent faulting and regional uplift are responsible for stream capture, stream drainage reversal, resorting of stream sediments, and modem alluvial placer formation. The volcanic-exhalative mineralization is hosted in metamorphosed low-K tholeiitic basalts, Ca-poor rhyolitic tuffs, and cherts. In the Fairbanks district the rocks are informally referred to as the Cleary sequence. Detrital zircons from the sequence yield U-Pb ages in the ranges 1.2, 1.3- 1.4, 1.8-1.9,2.5, and 3.4 Ga. The bimodal volcanic rocks are enriched in Au, Ag, As, Sb, and W. Average gold contents of the rocks exceed average crustal abundances by two orders of magnitude. Locally the metavolcanic rocks contain base metal massive sulfide mineralization with grades up to 20% combined Pb-Zn, 3 g/tonne Au, and 500 g/tonne Ag. These metavolcanic rock are correlated with those occurring in the Kantishna district (Spruce Creek sequence) and in the Circle district (Bonanza Creek sequence). The mineralized bimodal metavolcanic suite is thus shown to extend along strike for 350 krn (210 miles) through the Yukon-Tanana Terrane. In the Fairbanks district the Cleary sequence rocks are thrust over Type C eclogites. These eclogites trend northeasterly along the regional strike to the Circle quadrangle and are correlated with the eclogites of the central Yukon Territory. Lead 206/204 and 207/204 ratios from galena from the metavolcanic sequences and from the vein deposits are similar with average values of 19.10 and 15.69 respectively. The eclogitic rocks are less radiogenic with 206/204 and 207/204 ratios of 18.80 and 15.65 respectively. Low sulfide Au-quartz veins within the metavolcanic sequences are shown to be the product of multiple thermal and deformational events in the terrane taking place at 160-185, 140-145, and 90-125 Ma, K-Ar. Studies of the fluid inclusions in the metamorphic and vein quartz demonstrate that fluid compositions (1-20 mole % CO2; 3-5 wt % NaCl equiv.) and homogenization temperatures (275-375°C) are closely similar. Gold contents of the vein systems range from 5 to 18 g/tonne. Calc-alkaline plutons of Cretaceous (85-1 10 Ma) and Tertiary (50-70 Ma) age K-Ar host epithermal veins, Sn-greisen, and W-skarn mineralization, all of which are demonstrably gold-bearing. Rb-Sr initial ratios for the mineralized composite plutons are greater than 0.71 1 indicating that anatexis of the lower crust was the source of the granitic magma. The Cu-Mo-Au porphyry mineralization is hosted in the Tertiary plutons that intrude lower Palaeozoic and Mesozoic sediments of the North American Continental Margin (NACM) in the Tolovana district. The NACM rocks are separated from the metavolcanic sequence by the eclogitic rocks and by major thrust faults. Paleoplacer Au deposits hosted in continental clastic rocks of Eocene to Pliocene age are described. These have formed in small grabens adjacent to major strike-slip faults bounding the Yukon-Tanana Terrane on the northeast and southwest respectively. These structures, the Tintina and Denali Faults, controlled sedimentation and placer formation in these grabens. Using compilations of tonnage/grade data from examples of primary deposits analogous to those identified in the Yukon-Tanana Terrane, it is shown that a single large-scale deposit of any of these types could have supplied all the gold contained in the placer deposits of the region.